1. Field of the Invention
The subject invention relates to a supply assembly for supplying power to a light emitting diode (LED) lighting module.
2. Description of the Related Art
LED lighting modules are becoming more common in many applications for replacing less efficient incandescent lamps, for example, in traffic signal lights and automobile lighting. Depending on the amount of light required in the application, the LED lighting modules may consist of a plurality LED's arranged in parallel or in series, or a combination of both. In either case, the LED lighting module receives operating power from a supply assembly that switches a direct current voltage on and off at a high frequency. Such supply assemblies are known as switched-mode power supplies and are available in a plurality of forms, for example, a flyback converter, a buck converter, a half-bridge converter, etc. Each of these converters is capable of supplying a constant current to the LED lighting module in the form of a pulse width modulated signal.
In the use of LED lighting modules, it is desirable to be able to control the intensity of the light being output by the LED lighting module. This may be achieved in a number of ways. For example, the amount of current delivered to the LED lighting module may be adjusted by controlling the pulse width modulation. However, once the current intensity drops below 20% of the nominal current intensity, the relation between the current intensity and the light output becomes largely non-linear, and the efficiency of the LED lighting module becomes far from optimal.
U.S. Pat. No. 5,661,645 describes a power supply for a light emitting diode array which includes a circuit for interrupting the supply of power from the power supply to the LED array. As shown in FIG. 1 herein, the power supply 1 includes a supply of direct current voltage 10, which may be a battery or rectified line alternating current (AC) voltage connected to a switched-mode converter 12 typically having a control switch 14, a diode 16, an inductor 18, an optional capacitor 20 and an optional transformer 22. A control input of the control switch 14 receives a high frequency pulse-width modulated (PWM) switching signal. Outputs from the power supply 1 are connected to an LED lighting module 2 having an LED array 24 (shown herein as a single LED) and a controllable switch 26 for interrupting the supply of power to the LED array 24. The controllable switch 26 receives a low frequency PWM switching signal for controlling the mean current to the LED array 24. FIG. 2 shows a plot of the current through the LED array 24 in which the low frequency PWM switching signal causes current pulses D occurring in the period FD, and the high frequency PWM switching signal causes the current variation ΔID. While this arrangement ensures that the LED array always operates in an efficient manner, it should be understood that the power supply 1 is continually on even when the PWM switching signal has the controllable switch 26 turned off. FIG. 3 shows an equivalent circuit of the arrangement of FIG. 1. As should be apparent, while the power from the DC source is stopped when the control switch 14 is open, such is not the case when the controllable switch 26 is open. As such, this arrangement suffers from an unnecessary loss of energy.
Published U.S. Patent Application No. 2001/0023112A1 discloses an alternate arrangement to that shown in U.S. Pat. No. 5,661,645. In this alternate arrangement, the power supply itself is turned on and off using the low frequency PWM switching signal. FIG. 4 shows an example of this alternate arrangement. Similarly as in FIG. 1, the power supply 1′ includes a supply of direct current voltage 10, which may be a battery or rectified line alternating current (AC) voltage connected to a switched-mode converter 12′ typically having a control switch 14, a diode 16, an inductor 18, an optional capacitor 20 and an optional transformer 22. A control input of the control switch 14 receives a high frequency pulse-width modulated (PWM) switching signal. Outputs from the power supply 1′ are connected to an LED lighting module 2′ having an LED array 24 (shown herein as a single LED). The LED lighting module 2′ does not include the controllable switch 26 shown in FIG. 1. Rather, the switched-mode converter 12′ includes an input for receiving the low frequency PWM switching signal which effectively controls means for turning on and off the switched-mode converter 12′.